Add CHANGELOG.md entry for 2.1

[lkollar]

Preparing the new release: #181.

[codecov]

Codecov Report

Merging #184 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   87.64%   87.64%           
=======================================
  Files          19       19           
  Lines         963      963           
  Branches      210      210           
=======================================
  Hits          844      844           
  Misses         83       83           
  Partials       36       36

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